1. Field of the Invention
The present invention relates to elastic wave devices and specifically relates to elastic wave devices such as multi-band filters and duplexers that include piezoelectric substrates that face each other across a cavity.
2. Description of the Related Art
For example, as illustrated in the sectional view of FIG. 4, an elastic wave device (dual filter) has been proposed in which a cavity 118 is formed between piezoelectric substrates 112 and 114 by being joined to each other via an intermediate layer 113. The piezoelectric substrates 112 and 114, on which comb-shaped interdigital transducer (IDT) electrodes 121 and 123, which are for exciting surface acoustic waves, are formed, such that the surfaces on which the electrodes are formed face each other (See, for example, Japanese Unexamined Patent Application Publication No. 2008-546207).
In the case where a configuration in which piezoelectric substrates face each other across a cavity in this way is applied to a surface acoustic wave duplexer (SAW-DPX), forming a transmission (Tx) filter on one of the piezoelectric substrates and forming a reception (Rx) filter on the other of the piezoelectric substrates and then arranging the piezoelectric substrates such that the surface on which the Tx filter has been formed and the surface on which the Rx filter has been formed face each other has been considered. If such a configuration is adopted, a reduction in size can be achieved compared with a configuration in which both a Tx filter and an Rx filter are formed on one surface of a single piezoelectric substrate.
However, if a Tx filter and an Rx filter face each other with a small cavity therebetween, their isolation is degraded due to electromagnetic coupling therebetween. In order to improve upon this point, for example, as illustrated in the sectional view of FIG. 3, it is necessary that a Tx filter 21 and an Rx filter 23 respectively formed on piezoelectric substrates 12 and 14 be arranged in a staggered manner so as not to face each other across a cavity 18 formed between the piezoelectric substrates 12 and 14, which are connected to each other via a joining portion 13. However, if the positions of the Tx filter 21 and the Rx filter 23 are staggered in this way, the advantage of a reduction in size obtained with a configuration in which the two piezoelectric substrates 12 and 14 face each other across the cavity 18 is lessened.
Accordingly, in the case where a configuration in which piezoelectric substrates face each other across a cavity is applied to a SAW-DPX, it is difficult to both secure isolation characteristics and achieve a reduction in size.
In addition, when a configuration in which piezoelectric substrates face each other across a cavity is applied to a dual filter, in the case where the center frequencies of two filters are close to each other, out-of-band attenuation is degraded due to electromagnetic coupling of the two filters. Accordingly, it is difficult to both secure out-of-band attenuation and achieve a reduction in size.
The term “center frequency” is defined as the arithmetic mean of the lower side cutoff frequency and the higher side cutoff frequency in a band stop filter and the arithmetic mean of the lower side pass frequency and the higher side pass frequency in a band pass filter.